Typically, the negative electrode that is used in the lithium ion secondary battery is mainly constituted by a negative electrode active material layer and a current collector. The negative electrode active material layer is obtained by applying paste for manufacturing of a negative electrode, which contains a negative electrode active material such as a graphite material, a conductive auxiliary agent, a thickening agent, a water-based binder, and the like, onto a surface of a current collector such as copper foil (refer to Patent Document 1 and Patent Document 2) and drying the paste.
Here, when a non-uniform portion is present in the negative electrode active material layer, battery characteristics of a lithium ion secondary battery that is obtained may deteriorate. The non-uniform negative electrode active material layer may be easily obtained in a case where the paste for manufacturing of a negative electrode is not uniform. Accordingly, so as to obtain a uniform negative electrode active material layer, it is very important to uniformly disperse respective components such as the negative electrode active material, the conductive auxiliary agent, the thickening agent, and the water-based binder in the paste for manufacturing of a negative electrode.
Examples of a method of manufacturing the paste for manufacturing of a negative electrode include methods described in Patent Document 1 and Patent Document 2.
Patent Document 1 (Japanese Unexamined Patent Publication No. 2006-107896) discloses a method of manufacturing paste. The method includes at least three processes among a first kneading process of kneading graphite and a thickening agent in a powder state in combination with a dispersion medium, a dilution-kneading process of dilution-kneading the resultant kneaded object, which is obtained in the first kneading process, with the dispersion medium, and a finish kneading process of adding a binding agent to the resultant kneaded object that is obtained in the dilution-kneading process and kneading the resultant mixture to manufacture paste.
In Patent Document 1, a kneading shear force in the first kneading process is set to 2.5 times a kneading shear force in the dilution-kneading process and the finish kneading process. In Patent Document 1, it is described that highly dispersed and stable paste can be manufactured according to the setting of the kneading shear force.
Patent Document 2 (Japanese Unexamined Patent Publication No. 11-213990) discloses a method of manufacturing paste in which primary powderization of a solid content in paste is promoted by performing a mixing treatment with a strong shear force in a mixing process before addition of a binding agent resin, and agglomeration of the binding agent resin during mixing is prevented by performing a mixing treatment with a weak shear force in a mixing process after addition of the binding agent resin.
In Patent Document 2, it is described that a battery electrode, which has no pin-hole and crack on an active material layer and in which omission of the active material does not occur, can be obtained.